1. Field of the Invention
The present invention relates to a windmill-shaped loop antenna having a parasitic loop antenna; and, more particularly, to a windmill-shaped loop antenna having a parasitic loop antenna, which has an enough loop size to use a commercial probe while sustaining omni-directional pattern having the polarization purity of φ-polarization only by forming windmill-shaped metal patterns formed of loop pieces on a top and a bottom surface of a dielectric substrate and arranging the loop pieces of the top surface not to face the loop pieces of the bottom surface, and controls input impedance to match to system impedance by further including a parasitic loop antenna disposed at a predetermined distance from the windmill-shaped loop antenna.
2. Description of Related Art
In order to obtain an omni-directional pattern having the polarization purity of φ-polarization only, an antenna must have a structure to induce a magnetic dipole. A loop antenna may equivalently have the magnetic dipole characteristics. A small loop antenna having a short electric loop length of about λ/10 sustains the magnetic dipole characteristic.
The third and fourth generation mobile communication uses a frequency band of about 2 to 6 GHz. A small loop antenna for the third and fourth generation mobile communication is required to have less than 2.4 mm of a loop radius. Such a small loop antenna has a problem of using a commercial probe for power feeding due to the short loop radius of the small loop antenna.
The small loop antenna also has a problem of matching input impedance. That is, the small loop antenna has a bad antennal efficiency although a circuit for matching impedances is additionally used.
Therefore, there is a demand for an antenna structure that allows the physical length of loops and the impedance with an antenna radiation resistance to control while sustaining an omni-directional small loop antenna pattern with φ-polarization only.
According to a conventional loop antenna technology, a loop antenna having a loop rolled up several times was introduced. Such a rolled-up loop increases the radiation resistance and performs impedance matching. However, the conventional loop antenna with the rolled-up loop has problems of reducing the polarization purity and breaking the omni-directional pattern.
According to another conventional loop antenna technology, another loop antenna using coaxial cable pieces was introduced to only obtain the φ-polarized pattern regardless of the electric length of the loop. However, it is difficult to embody the conventional loop antenna with coaxial cable pieces to be operated at a frequency higher than 2 GHz and has the limitation for impedance matching because the conventional loop antenna with coaxial cable pieces is not a thin film structure.